Exercise Metabolism

Oxygen deficit and steady state VO2

• Oxygen deficit refers to the delay in oxygen uptake at the onset of exercise.

• Steady state VO2 is the plateau in oxygen uptake which is attained within 1- 4 minutes of constant exercise.

VO2 response of trained vs. untrained subjects

• Trained subjects will reach steady state faster than untrained subjects.

• This is likely because trained subjects have improved aerobic response to exercise stimulus.

Oxygen deficit and debt/EPOC during light and heavy exercise

• The term “oxygen debt” should not be used in exercise physiology.

• Excess Post-exercise Oxygen Consumption (EPOC) is a more correct term for the elevated VO2 seen after exercise.

Oxygen deficit

• O2 deficit occurs at the onset of exercise

• The workload is greater than aerobic metabolism can support– Takes time to increase aerobic metabolism from

rest to exercise load

• The difference between the metabolic need and the aerobic contribution is the O2 deficit

Steady State

• Steady state is reached once the aerobic metabolism meets the workload demand

• The workload must remain constant

• If the workload is greater than VO2max, steady state will never be reached

Oxygen Debt

• Following the exercise bout, O2 consumption is elevated for a period of time

• This is referred to as O2 debt– Paying back the deficit

• The debt is composed of a rapid and a slow component

Potential factors contributing to O2 debt (fast)

• Resynthesis of PC

– synthesized rapidly within minutes

• Restoration of muscle and blood O2 stores

– restored more quickly than PC stores

Potential factors contributing to O2 debt (slow)

• Lactate removal

– Possible

• Elevated body temperature

– Possible-may be elevated for prolonged period

• Post-exercise elevation in HR and breathing

– Probably

• Elevations in hormones (epinephrine, norepinephrine)

Removal of Lactate Following Exercise

Metabolic Response to Short-Term Intense Exercise

• High-intensity, short-term exercise (0-10 seconds)– ATP production through ATP-PC system

• Intense exercise longer than 20 seconds– ATP production via anaerobic glycolysis

• High-intensity exercise longer than 60 seconds– ATP production primarily through oxidative

mechanisms

Metabolic Response to Prolonged Exercise

• Exercise longer than 10 minutes– ATP production primarily from aerobic

metabolism– Steady state oxygen uptake can generally be

maintained

• Prolonged exercise in a hot/humid environment or at high intensity– Steady state not achieved– Upward drift in oxygen uptake over time

VO2 drift

Metabolic Response to Exercise: Incremental Exercise

• Oxygen uptake increases linearly until VO2max is reached– No further increase in VO2 with increasing

work rate

• Physiological factors influencing VO2max

– Ability of cardiorespiratory system to deliver oxygen to muscles

– Ability of muscles to take up the oxygen and produce ATP aerobically

Incremental exercise and VO2max

Lactate Threshold

• the lactate threshold is an inflection point where concentrations in the blood increase exponentially with an increase in workrate

• it will often correspond to the ventilatory threshold where an individual’s breathing becomes labored

Incremental exercise and blood lactate concentration

Potential mechanisms to explain the lactate threshold phenomenon

Potential Contributors to the Lactate Threshold

• low muscle oxygen– as the individual exercises at a higher intensity,

oxygen availability in the muscle is insufficient to meet the “aerobic” system demands

– this produces a back log and pyruvate must be converted to lactate

– this is an unlikely scenario as oxygen delivery and utilization is quite efficient

• accelerated glycolysis– as the increasing energy demands of intense

work cannot be met by “aerobic” glycolysis-Krebs alone, glycolysis is accelerated in order to produce a little more energy (ATP)

Failure of the hydrogen shuttle to keep pace can increase lactate concentration

• recruitment of fast twitch fibers– fast twitch vs slow twitch fibers– slow twitch used for low intensity “aerobic”

activities– fast twitch for hi intensity “anaerobic” activities

– slow twitch readily convert lactate back to pyruvate (LDH)

– fast twitch readily covert pyruvate to lactate (LDH)

Fuel Utilization During Exercise can be Determined by Gas

Exchange

• RER = VCO2/VO2

• that is the ratio of CO2 produced to O2 consumed

• CO2 production is an estimate of the relative contribution of glycolysis to energy production– glucose >> pyruvate >> acetic acid + CO2

>>>> Krebs >>> O2 + H20

• when fat enters Krebs as acetyl coA no CO2 is produced, BUT CO2 is formed in Krebs

Percentage of fat and carbohydrate metabolized as determined by a nonprotein Respiratory Exchange

Ratio (RER)

RER % Fat % Carb

.70 100 0

.75 83 17

.80 67 33

.85 50 50

.90 33 67

.95 17 831.00 0 100

Intensity vs Fuel Source

• at higher intensity workloads, carbohydrate will become the primary fuel source

• conversely, at lower workloads, fats will predominate

The crossover concept of metabolism vs. intensity

Duration vs Fuel Source

• for short duration or during the early stages of prolonged exercise, carbohydrates will be the primary fuel source

• for longer duration fat will become the predominate source

Fuel shifts from CHO to fat during prolonged exercise

Sources of Fuel During Exercise

• Carbohydrate

– Blood glucose

– Muscle glycogen

Fat

• Plasma FFA (from adipose tissue lipolysis)

• Intramuscular triglycerides

Protein

• Only a small contribution to total energy

production (only ~2%)

– May increase to 5-15% late in prolonged

exercise

Blood lactate

• Gluconeogenesis via the Cori cycle

Muscle fuel sources in highly trained endurance athletes

Fuel Sources in Highly Trained Athletes

• Low Intensity

– Primary source plasma FFA (from fat stores)

• Middle Intensity

– Equal from plasma FFA and muscle glycogen

• High Intensity

– Muscle glycogen predominate source

Contributions of four energy sources over prolonged time in endurance athletes

Prolonged Exercise in Trained Endurance Athletes (70 % VO2max)

• Less than 1 hour glycogen is primary single source– FFA and muscle triglycerides comprise 50 % though

As exercise progresses

• Contribution from glycogen reduced

• Muscle triglycerides (intramuscular fat) reduced also

• FFA and plasma glucose increase• Must feed to maintain plasma glucose

The Cori cycle: lactate as a fuel source